1. Field of the Invention
The present invention relates to a method and apparatus for storing data in an optical storage medium, reading the data from the optical storage medium, retrieving the data from the optical storage medium, and also relates to the optical storage medium used for such data storing, reading and retrieving.
2. Discussion of the Related Art
A rewritable optical disk such as a phase change disk or a magneto-optical disk has already used widely. The storing density of the optical disk is larger than that of a general magnetic disk by at least one digit. However, it is still insufficient for digital storage of image information. To enhance the storing density, it is necessary to reduce the beam spot diameter to shorten the distance to the adjacent track or bit.
A DVD-ROM is put into practice by such development of technique. The DVD-ROM with 12 cm diameter can store 4.7 GByte data on one side. A writable/erasable DVD-RAM with 12 cm diameter can realize high-density storage of 5.2 GByte data on both sides, which is more than 7 times as large as the capacity of a CD-ROM and corresponds to the capacity of more than 3,600 floppy disks.
The optical disk has been improved to obtain higher density and larger capacity from year to year. However, since the optical disk stores the data in the two-dimensional surface, the storing density is restricted by the light diffraction limit and is nearing 5 Gbit/cm2. To obtain larger capacity, three-dimensional storage (volume holographic storage) further utilizing a depth direction is required.
Materials for the three-dimensional (volume holographic) optical storage medium are, for example, a photopolymer material, photorefractive material and the like. Since these materials change their refractive indexes by absorbing relatively week light beam, it is possible to use the change of the photo-induced refractive index for storing information. Therefore, these materials can be used for multiplexed holographic storage that realizes the larger capacity.
An example of high-density storage utilizing the photopolymer material is discussed in xe2x80x9cSPIE Vol. 2514,355xe2x80x9d. Shift-multiplexed holograms are stored in a disk, that is made from DuPont""s 150-100 photopolymer and rotated, using a spherical wave as a reference beam. As a result, the storing density of 10 times as large as that of a CD currently used, 10 bit/xcexcm2, is obtained.
An example of high-density storage using the photorefractive material is described in xe2x80x9cOPTICAL ENGINEERING Vol. 34, 2193 (1995)xe2x80x9d. It is reported that 20,000-page holograms are multiple-stored in Fe-doped LiNbO3 crystal of the size of 10xc3x9710xc3x9722 mm, and thereby about 1 -GByte data storage is achieved.
The holographic memory can store the large capacity of data as described above, and in addition, it can write and read the pieces of data disposed two-dimensionally. Accordingly, it is possible to perform high-speed data storing, reading, retrieving, correlation detection and transfer by using the holographic memory. Specifically, the following data retrieving method is disclosed by Japanese Patent Application Laid-Open No. 3-149660 (1991).
FIG. 26 shows a device for retrieval. A laser 101 emits a laser beam to read pieces of two-dimensional retrieving object data holographically stored from an optical memory 102. A data pattern image is written to a spatial light modulator 103 of the optical address type. Two-dimensional retrieving data is written to a spatial light modulator 104 of the electric address type that is a liquid-crystal display (LCD) panel.
The spatial light modulator 104 is illuminated with a laser beam as a read beam from a laser 105 through an analyzer 106. The polarization state of the beam is changed in accordance with the retrieving data, and the light transmitted through the spatial light modulator 104 is reflected off a half-mirror prism 107. Then the light forms an image on a readout surface of the spatial light modulator 103 of the optical address type.
Thus, the polarization state of the read beam is modulated by the spatial light modulator 103 for each pixel in accordance with the retrieving object data. The read beam illuminates a photodetecting array 109 through an analyzer 108, and the photodetecting array 109 performs batch detection as to whether there is any read beam transmitted through the pixels. Thus the batch detection of matching between bits of the retrieving object data and those of the retrieving data is possible.
xe2x80x9cConjugate Image Plane Correlator with Holographic Disk Memoryxe2x80x9d, A. Kutanov and Y. Ichioka, OPTICAL REVIEW Vol. 1.3, No. 4, 1996, pp. 258-263 describes a data storage method and data correlation detecting method as follows.
FIG. 27 shows a device used in the storage method and correlation detecting method. In storing the data, two-dimensional data to be stored is displayed on a spatial light modulator 111 of the electric address type that is an LCD panel. A signal beam 112 having a two-dimensional amplitude modulation transmitted through the spatial light modulator 111 is Fourier transformed on a Fourier plane P1 by a lens 113 and illuminates an optical memory 114. At the same time, a reference beam 115 illuminates the optical memory 114 and the two-dimensional data is stored as a Fourier-transform hologram in the optical memory 114.
In detecting correlation, the two-dimensional retrieving data is displayed on the spatial light modulator 111 of the electric address type, and in addition, a read beam 116 having conjugate relation with the reference beam 115 used in storing illuminates the optical memory 114. The diffracted beam of the two-dimensional retrieving object data is read out from the hologram stored in the optical memory 114, and the diffracted beam is transformed on a Fourier plane P2 by the lens 113. Then the beam illuminates the spatial light modulator 111.
Accordingly, the transmitted beam from the spatial light modulator 111 is an optical product of the retrieving data and the retrieving object data. If the retrieving data and the retrieving object data match with each other, a strong correlation peak appears on a Fourier plane P3 through a lens 117. By detecting the peak, correlation between two-dimensional images can be found.
As an optical storage medium in which the hologram can be rewritten, an optical storage medium made of liquid crystal polymer is disclosed by Japanese Patent Application Laid-Open No. 2-280116 (1990), and an optical storage medium made of a phase change material is disclosed by Japanese Patent Application Laid-Open No. 4-30192 (1991).
As described so far, attentions have recently been paid to the holographic memory to improve the memory capacity and processing speed, and the retrieving method discussed with reference to FIG. 26 and the storage method and the correlation detecting method discussed with reference to FIG. 27 have been proposed. Furthermore, enhancement of the signal-to-noise ratio (S/N) has been researched to realize high-density storage.
However, the conventional retrieving method, storage method and correlation detecting method explained with reference to FIGS. 26 and 27 have adopted a spatial light modulator of an amplitude (intensity) modulation type that is an LCD panel 104 or 111. Therefore, the following problems have been caused.
As shown in FIG. 28, like the spatial light modulators 104 and 111, an LCD for displaying data is constructed by forming a liquid crystal cell 124 containing a liquid crystal 121 and electrodes 122 and 123 on both sides of the liquid crystal 121 and disposing polarizers 126 and 127 on the outside of the liquid crystal cell. Dichromatic polarizers are used as the polarizers 126 and 127 because they can be downsized easily. However, since the transmittnace of the dichromatic polarizer in the direction of transmission axis is as low as 70-80%, if two dichromatic polarizers are used together, 50% transmission loss is caused.
Therefore, in the case where data storing and reading are performed by utilizing the spatial light modulator that is an LCD panel, the light intensity is reduced in both of the storing and reading processes, and thereby S/N is also reduced. As a result, deterioration of hologram storing density or retrieving precision occurs. If the laser power is raised for increasing the signal intensity, a life span of the laser is shortened.
In storing and reading the data utilizing the holographic memory, there are the following noise factors which determine a bit error rate (BER):
(1) noises irrelevant to the quality of the hologram caused by a photodetecting array such as a CCD or the like;
(2) diffracted light from the adjacent hologram (crosstalk between pages);
(3) crosstalk between pixels in a reconstructed image; and
(4) fluctuation of diffraction efficiency in a single page or between pages caused by the defect of a crystal or optical system.
Information storage utilizing the amplitude (intensity) modulation is apt to be affected by various noises, and the storing density of the storage medium depends on the signal to the noise ratio (S/N). Therefore, several coding attempts have been made to restrict BER in the same way as other filing systems.
In the case where pieces of two-dimensional data having correspondence of [clear, dark] to [0, 1] are multiple-stored in holograms, a crosstalk resulting from the fluctuation of the diffraction efficiency occurs because the whole light intensity of the signal beam used in storing cannot be constant according to the data. To avoid the problem, a differential encoding method is adopted, in which [dark-clear] corresponds to [0], and [clear-dark] corresponds to [1]. However, in this case, the coding ratio is 0.5, in other words, using efficiency of pixels is decreased.
As described above, if the spatial light modulator of the amplitude modulation type is employed to input data or retrieve data, some problems arise such as a low light using efficiency, deterioration of S/N, need for a special encoding, and so forth. Consequently, high-density storage, which is one of the characteristics of the holographic memory, cannot be sufficiently achieved in fact.
Moreover, the conventional data retrieving method explained with reference to FIG. 26 has problems in that:
(1) an expensive spatial light modulator 103 of the optical address type is required;
(2) a highly precise alignment of the spatial light modulator 103 of the optical address type and the spatial light modulator 104 of the electric address type is required;
(3) storage of a hologram in the optical memory 102 requires another spatial light modulator, and so forth.
The conventional storage method and correlation detecting method described with reference to FIG. 27 can avoid the above problems (1) to (3). However, since the detection of correlation between the data depends upon whether a correlation peak exists, a serious problem occurs. That is, it is impossible to detect matching between a bit of data and that of other data that are complex and of high-density, though a correlation value between pieces of data can be obtained. Therefore, these methods are not suitable for a computer filing system capable of retrieving.
It is possible to rewrite the hologram by using photo-refractive materials such as Ba2TiO3, LiNbO3, SBN (SrxB1xe2x88x92xNb2O6), or the liquid crystal polymeric material disclosed in Japanese Patent Application Laid-Open No. 2-280116 or the phase change material disclosed in Japanese Patent Application Laid-Open No. 4-30192.
However, the conventional optical storage medium and optical storage method using thereof, in principle, cause some changes in the materials in a portion of high light intensity, and in contrast, do not cause any change in the materials in a portion of low light intensity. Therefore, if it is desired to rewrite the data without an erasing process, a problem occurs. Suppose that, in a region, the content of the previously stored data causes changes in materials by high light intensity and the content of the new data does not cause changes in materials because of low light intensity. In the region, the content of the data that has caused changes in materials remains, and as a result, it is impossible to rewrite the data.
Consequently, when the data is to be rewritten, it is necessary to erase the previously stored data by an erasing process such as illuminating the whole surface of the optical storage medium with a laser beam and to write the new data. It takes so much time to rewrite the data, and accordingly, high processing speed, that is one of advantages of the holographic memory, is lost.
The present invention has been made in view of the above circumstances and has an object to provide an optical storage apparatus and optical storage method using thereof which can perform high-density storing at high speed and can rewrite data at high speed without an erasing process.
Another object of the present invention is to provide an optical reading apparatus and optical reading method using thereof which can read out data stored in an optical storage medium with very high precision at high speed.
Further object of the present invention is to provide an optical retrieving apparatus and optical retrieving method using thereof which can retrieve a necessary piece of data with high precision at high speed from an optical storage medium in which a large amount of data is stored.
Further object of the present invention is to provide an optical storage medium suitable to high-speed storing, reading, retrieving data with high precision, and high speed rewriting of data without requiring an erasing process.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, an optical storage medium of the present invention is a polarization-sensitive member that have a photo-induced birefringence property. The optical storage medium of the present invention may also be a sheet-like light-transmitting material which has the polarization-sensitive member with the photo-induced birefringence property at least on one side as a layer.
The optical storage method of the present invention provides a signal beam that retains spatial polarization modulated data modulated by a spatial light modulator capable of modulating polarization of a light beam. The signal beam and a reference beam simultaneously illuminate an optical storage medium for storing a hologram of the polarization modulated data retained by the signal beam in the optical storage medium.
In the optical reading method of the present invention, a read beam illuminates an optical storage medium storing a hologram generated cooperatively by a reference beam and a signal beam retaining spatial polarization modulated data. The data is then read out based on a polarization modulation of a diffracted beam from the hologram.
In the optical retrieving method of the present invention, a read beam illuminates an optical storage medium storing a hologram generated cooperatively by a reference beam and a signal beam retaining spatial polarization modulated data as retrieving object data. A diffracted beam from the hologram is transmitted through a spatial light modulator that modulates polarization of a light beam in accordance with retrieving data. Matching between the retrieving object data and the retrieving data can be detected based on the polarization modulation of the transmitted diffracted beam.
In a conventional holography, a light intensity modulation based on an interference pattern of a signal beam and a reference beam is recorded as a change of a refractive index or an absorption in an optical storage medium. Accordingly, it is necessary that the polarizing directions of the signal beam and reference beam are in parallel. Amplitude and phase of the signal beam can be stored, but storage of polarizing direction is limited to only one direction. Therefore, in conventional holographic storage or data retrieval, a spatial light modulator of an amplitude modulation type has been used as described above.
In contrast, a material showing photo-induced birefringence (also referred to as photo-induced dichroism or photo-induced anisotropy) senses a polarization state of a light beam incident thereon, and is able to store a polarizing direction of the incident beam. As described later, inventors of the present invention have found materials having particularly excellent storage characteristics as a result of their researches and experiments.
Paying attention to the point, the present invention constructs an optical storage medium by making a sheet from a light-transmitting material and forming a polarization-sensitive layer having the photo-induced birefringence at least on one side of the sheet. Hereinafter, such optical storage medium according to the present invention is referred to as polarization-sensitive optical storage medium.
The polarization-sensitive optical storage medium can store a hologram generated by the photo-induced birefringence corresponding to the polarization modulation by an interference pattern of two light waves when the polarizing directions of the signal beam and reference beam are orthogonal to each other. In this specification, such hologram is referred to as polarization hologram in contrast with the hologram recorded by usual light intensity modulation. By illuminating the polarization hologram with a read beam having the same polarizing direction as that of the reference beam used in recording, a diffracted beam retaining the polarizing direction of the signal beam is available.
Paying attention to this point, the optical storage method is devised to obtain the signal beam retaining data information based on a spatial polarization modulation by a spatial light modulator capable of modulating polarization, illuminate the optical storage medium with the signal beam and reference beam at the same time, and thereby store the polarization modulation of the signal beam as a hologram in the optical storage medium. The optical reading method of the present invention illuminates the optical storage medium with a read beam, in which the signal beam retaining the data information based on the spatial polarization modulation is stored as the hologram by the reference beam, and thereby reads out the data information in accordance with the polarization modulation of the diffracted beam from the hologram.
Also, paying attention to the above point, the optical retrieving method according to the present invention illuminates the optical storage medium with the read beam, in which the signal beam retaining pieces of retrieving object data owing to the spatial polarization modulation is stored as the hologram by the reference beam. The diffracted beam from the hologram illuminates the spatial light modulator that modulates polarization in accordance with the retrieving data. Based on the polarization modulation of the light transmitted through the spatial light modulator, matching between the retrieving data and the retrieving object data is detected.
Since the spatial light modulator capable of modulating polarization can be constructed without a polarizer, there is no light transmission loss. Moreover, the signal beam retains the data information in the form of the spatial polarization modulation; therefore the light intensity modulation of the signal beam is constant. Consequently, the optical storage method according to the present invention can prevent the deterioration of S/N of the signal beam, and thereby can store the data with high precision at high speed.
Therefore, according to the optical storage method of the present invention, the data stored in the optical storage medium can be read out with high precision at high speed. The optical retrieving method of the present invention makes it possible to easily retrieve the required data from the optical storage medium that stores a large amount of data with high precision at high speed.
Furthermore, as described later, as a result of the research and experiments, the inventors of the present invention have found that, if data is stored as a polarization hologram in an optical storage medium by the optical storage method of the present invention, new data can be overwritten as another polarization hologram on the optical storage medium according to the optical storage method of the present invention without erasing the previously stored data by an erasing process such as radiation of laser beam over the whole surface of the optical storage medium.
Accordingly, the above-described optical storage method of the present invention enables high speed rewriting of data without an erasing process.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention.